Update diffmerge

In reference to #5133, this update to `diffmerge` symlinks the `DiffMerge` binary, instead of the devloper provided shell script. This has the advantage of working (the developer provided shell script assumes DiffMerge.app lives in /Applications) but the disadvantage of causing a wxWidgets Debug alert to appear when launching `diffmerge` from the command line. This method also misses out on a `--nosplash` option from the developer provided shell script.

Unfortunately I don't know why this method causes the wxWidgets debug alert when the previous one doesn't.